1. Field of the Invention
The present invention relates to an evaluation device of an optical writing device and an evaluation method of the optical writing device.
2. Description of Related Art
In recent years, an image forming apparatus has been developed in which an LED print head (hereinbelow referred to as an LPH), as an optical writing device to form an electrostatic latent image on a surface of a photo conductor. The LPH comprises an LED chip array, and a lens array. The LED chip array is configured in a state where an LED chip comprising a plurality of LED (Light Emitting Diode) elements are arranged along the main scanning direction according to a previously set resolution on an array. The lens array is configured in a state where a plurality of GRIN (Graded-Index) lenses are arranged to collect irradiation light from LED elements which emit light according to image data, so that an electrostatic latent image is formed with an image on a photo conductor.
It is known that light amount unevenness is generated while the light writing processing using the LPH because of the manufacturing variability of the LED elements; the variability in optical characteristics caused by the mounting position, the change of angles, the refractive index distribution, and the like, of the GRIN lens; the attachment of dust, and the like. In an image forming apparatus using such LPH, density unevenness is generated at places where the light amount unevenness has been generated, therefore there is a problem that noise lines such as white noise lines, black noise lines, and the like, are generated because of the density unevenness. These noise lines are generated in an image as shadows in a form of lines by the density unevenness, thus may also be referred to as shadow noise lines.
Japanese Patent Application Laid-open Publication No. 2006-248185 discloses an optical writing device to previously specify the position of the noise lines caused by the head or the lens array so as to correct only the noise lines. In this optical writing device, places in need of correction where the linearity of the image density with respect to the light amount variation of the image data input to the light exposing head is lost is specified by the MTF (Modulation Transfer Function) of the lens array or the MTF of the light exposing head, thereby the specified places in need of correction is subjected to the correction of the density variability.
However, in the technique disclosed in the Japanese Patent Application Laid-open Publication No. 2006-248185, when the places in need of correction is to be specified, the defocusing position is changed by 5 levels to observe the variation of the MTF at each dot position, thereby the places where the MTF drastically changes is specified, on the graph in which the horizontal axis represents the dot position and the vertical axis represents the MTF. Thus, in the technique disclosed in the Japanese Patent Application Laid-open Publication No. 2006-248185, places in need of density correction are specified according to the MTF characteristics at a plurality of different defocusing positions, and there is a possibility that the specified places in need of correction may be different from the generation position of the noise lines which is supposed to be generated at a position where the image is formed. Further, in the technique disclosed in the Japanese Patent Application Laid-open Publication No. 2006-248185, the places in need of correction are specified in the image forming apparatus which comprises the LPH, thus when there is a serious image quality issue, there is a possibility that the issue cannot be handled only by the correction processing performed in the image forming apparatus